Method of manufacturing heat resistant wires



United States Patent 3,226,823 METHOD oF MANUFACTU ING HEAT R SIS A T WIRES Hans Von Kantzow, Ramnas, Sweden, assignor to Aktiebolaget Kanthal, Hallstahamma-r, Sweden, a corporation of Sweden No Drawing. FiledSept. 10, 1963, Ser. No. 307,789 7 Claims. (Cl. 29-528) This invention relates to a method: of manufacturing heat resistant wires,,particularly for use in electric resist ance heating elements.

Electric resistance heating elements are, manufactured from alloys possessing the ability to withstand oxidation at high temperature in air. It is a well established fact that this feature is found with the iron-chromium-aluminium alloys which may be used at elevated temperatures and at the same time, have a considerable period of service. A great number of alloys of this kind have been proposed previously. It is also true that the methods of manufacturing heat resistant wires from such alloys are highly developed, so that very uniform products are obtained. However, although quite satisfactory for several purposes, the method of manufacturing heat resistant wires, particularly for use in electric resistance heating elements as hitherto known, may be improved in certain respects. In particular, it is desirable to develop a method of manufacturing wires which combine great tenacity, workability, heat resistance and endurance with a high maximum continuous service temperature, and which have very constant and uniform physical properties.

Accordingly, the primary object of the present invention is to providea method of manufacturing resistance wires, which meet all of the above recited requirements simultaneously and to a surprisingly high extent.

The method according to the present invention is characterized by the steps of making a melt containing:

Percent by weight The remainder being iron,

forming said melt, such as by casting and, if desired, a subsequent forging step, into elongated blanks or billets for hot-rolling, hot-rolling said elongated blanks or billets into wires with a diameter of about 6-8 millimetres at a temperature of 1175-1290 C., and allowing the same to cool down to room temperature, and cold-forming said strips or wires, as by cold-drawing to the finally desired dimension in a plurality of successive fold-forming steps, if desired with intermediate annealing steps.

It has been ascertained that an alloy as recited above exhibits a very great workability resulting in a high yield at the manufacture, when cold-drawing of wires is applied, as well as considerably reduced manufacturing costs as compared to those for the hitherto known iron-chromi- V um-aluminium alloys. Also, a surprisingly great heat resistance is obtained, which is dueto the fact that a highly resistant superficial oxide layer is formed on the surface of the wire used in an oxidising atmosphere. The tenacity is great enough to ensure that resistance wiresof the alloy are capable of withstanding all mechanical stresses as normally encountered. Moreover, the physical and mechanical properties of the alloy in question are very constant and uniform, which, of course, is very desirable to en- 3,226,823 Patented Jan. 4, 1966 c ICC able accurate calculations in the design as well as a reliable operation of resistance heating elements of the alloy in question.

Depending on the various compositions within the above mentioned limits the maximum continuous temperature for resistance wire elements made of an alloy of this general class is within the range of 1150-1350 C. (2102- 2462 F.), the resistivity at 20 C. (68 F.) ranges from to microhm per cm. (812-872 ohms per cir. mil foot). The specific gravity is 7.25-7.1 and the coefficient of linear expansion varies from 10.5 x10 within the range 20-250 C. (68-482 F.) to 14.0 10- within the range 20:-l000 C. (68-1832 F.). The heat conduction at 20 C. (68 F.) lies at 0.04 cal./cm. x sec. x C. the specific heat at 0.11 cal./g. x C., and the melting point at approximately 1510 C. (2750 F.). The Brinell hardness is 200-260, the elongation 12-20% for 200 mm. (8"), and the yield point is 45-65 kp./mrr1. (64.000- 92. 000 lbs. per sq. inch). The tensile strength is 65-66 kp./cm. (92000-121000 lbs. per sq. inch), but rises with decreasing dimensions so as to attain a five to ten percent higher value at small wire or strip dimensions.

One group, of alloys within the above mentioned limits may have a chromium content of 20-24%. Taking one example within this first mentioned group, a resistance wire made of an alloy having an aluminum content of 4.0-4.9% may be used at a maximum element temperature of about 1200 C. (2192 F.) and has for instance, when used for industrial furnace elements for the most economical life, a permissible surface lead of about 2.5 W/cm. at a furnace temperature of 900 C. (1652 F.). Of course, this figure for the permissible surface load varies greatly, and it is dependent on the design of the element, the supports therefor and the furnace as well as atmosphere, the frequency of switching etc. The same considerations are applicable also to the surface load figures as mentioned below.

Taking another example within the first mentioned group with a chromium content of 20-24%, a resistance wire element made of an alloy of this general composition having an aluminum content of 50-53% may be used at a maximum temperature of about 1300" C. (2371 F.). This resistance wire, when used for electrical resistance heating elements for industrial furnaces, has for the most economical life a permissible surface load range from 1.6-3.0 W/cm. at 1000 C. (1832 F.) furnace temperature up to 1-1.7 W/cm. at 1200 C. (2192 F.) furnace temperature.

As a third example, a resistance wire element made of an alloy belonging to the first mentioned group, i.e. with a chromium content of 2024%, and having an aluminium content of 5.4-6.5 may be used at a maximum tem perature of about 1350 C. (2462 F.). A surface load of from 1.5-2.5 W/cm. at 1150 C. (2052 F.) furnace temperature to 1.0-1.5 W/cm. at 1300 C. (2372 F.) is economically possible with industrial furnace elements of this last mentioned alloy.

The tenacity, which is essential to ensure the necessary mechanical resistance, may be further improved by adding 0.8-1.2% Ta and 0.1-0.3% Nb to the alloy. In this case, the maximum continuous service temperature amounts to about 1200 C. (2192 F.).

Another group of alloys of the above mentioned general class has a slightly less chromium content, viz. 13- 16%, whereas the Si-content is 0.3-0.8% and the lowermost limit of the Mn-content is increased from 0.2% to 0.3%. Ingeneral, the properties of the just mentioned type of alloy corresponds to those of the above exemplified alloys.

It should be pointed out that it has been found essential to have a minimum content of 0.01% Zr when the alloy is to the drawn into resistance heating wires.

Of course, this invention is by no means restricted merely to the method proper but should moreover be considered to include within the scope of protection all resistance heating wires, elements and similar products made therefrom, whatever shape such products may have. Also, it is to be understood, that the above compositions are to be taken in their broadest sense, in as much as several modifications thereof are possible within the prin' ciples of the invention.

What I claim is:

1. A method of manufacturing heat resistant wires, particularly for use in electric resistance heating elements,

which comprises the steps of making a melt containing Percent by weight The remainder being iron,

forming said melt by casting and a subsequent forging step, into elongated blanks for hot-rolling, hot-rolling said elongated blanks or billets into wires with a diameter of about 6-8 millimetres at a temperature of 11751290 C.

and allowing the same to cool down to room temperature, and cold-forming said wires by cold-drawing to the finally desired dimension in a plurality of successive colddrawing steps.

2. A method as claimed in claim 1 wherein a melt with a chromium content of 20-24% is used.

3. A method as claimed in claim 2 wherein a melt with an aluminium content of 4.0-4.9% is used.

4. A method as claimed in claim 2 wherein a melt with an aluminium content of 5.0-5.3% is used.

5. A method as claimed in claim 2 wherein a melt with an aluminium content of 5.4-6.5 is used.

6. A method as claimed in claim 2 wherein a melt having an addition of 0.8-1.2% Ta and 0.1-0.3% Nb is used.

7. A method as claimed in claim 1 wherein a melt containing 13-16% chromium, 0.30.8% Si and 0.3% Mn as a lowermost limit is used.

References Cited by the Examiner UNITED STATES PATENTS 1,901,514 3/1933 Herman et al 803 2,357,342 9/1944 Montgomery 8063 2,703,355 3/1955 Haaglund 124 OTHER REFERENCES.

Metals Handbook, Volume I, Eighth Edition, pages 424 to 426.

MARCUS U. LYONS, Primary Examiner. 

1. A METHOD OF MANUFACTURING HEAT RESISTANT WIRES, PARTICULARLY FOR USE IN ELECTRIC RESISTANCE HEATING ELEMENTS, WHICH COMPRISES THE STEPS OF MAKING A MELT CONTAINING 